A touch panel has been widely known as a device that detects a position (hereinafter may be referred to as a “touch position”) indicated by an indicator, such as a finger of a user and a pen, on a touch screen and outputs the touch position. A plurality of detection techniques of a touch panel for detecting a touch position are known. One of electrical capacitance touch panels is a projected capacitive touch panel.
The projected capacitive touch panel can detect a touch position in a case where a surface of a touch screen on a user side (hereinafter may be referred to as a “front-side surface”) is covered with a protective plate such as a glass plate having a thickness of approximately several millimeters. The projected capacitive touch panel has several advantages such as excellent ruggedness due to the placement of the protective plate on the front-side surface and a long life due to the absence of any movable part.
A touch screen of the projected capacitive touch panel is formed of detection row-direction wiring for detecting coordinates of a touch position in a row direction and detection column-direction wiring for detecting coordinates of the touch position in a column direction (for example, see Patent Document 1). In the following description, the detection row-direction wiring and the detection column-direction wiring may be collectively referred to as “detection wiring.”
Patent Document 1 discloses a touch pad system corresponding to the touch panel. The touch pad system disclosed in Patent Document 1 includes, as the detection wiring for detecting an electrostatic capacitance (hereinafter may be simply referred to as a “capacitance”), a first series of conductive elements formed on a thin dielectric film and a second series of conductive elements formed above the first series of conductive elements with an insulating film therebetween. These series of conductive elements do not electrically contact each other. One group of the first series of conductive elements and the second series of conductive elements when viewed in the direction of the normal to the front-side surface overlaps the other group, forming intersections without electrically contacting each other.
A detection circuit detects a capacitance (hereinafter may be referred to as a “touch capacitance”) formed between an indicator such as a finger and the conductive elements as the detection wiring, to thereby specify coordinates of a touch position of the indicator. The touch position between the conductive elements can be interpolated by a relative value of a detection capacitance of one or more conductive elements.
In the following description, a member being a transparent dielectric substrate on which the detection column-direction wiring and the detection row-direction wiring are located is referred to as a “touch screen,” and a device being the touch screen connected to the detection circuit is referred to as a “touch panel.” In the touch screen, a region where a touch position can be detected is referred to as an “operation region” or a “detectable area.”
To thoroughly detect a touch position of an indicator in the operation region of the touch screen, the detection wiring needs to be disposed closely to each other in the operation region. The detection wiring being disposed closely to each other in the operation region in such a manner needs to be prevented from being visually identified by the user.
The detection wiring made of a transparent conductive film such as indium tin oxide (ITO) is less likely to be visually identified by the user. However, the transparent conductive film such as the ITO has a relatively high electrical resistance (hereinafter may be simply referred to as a “resistance”), resulting in the disadvantage of increasing the touch screen in size. Moreover, corrosion of the transparent conductive film such as the ITO relatively easily occurs between the other metal wiring and the transparent conductive film, causing a break in the wiring. For the use of a liquid crystal display (LCD) on which the touch screen is mounted, stability against humidity and water droplets is the challenge.
A metal material having a low resistance, for example, silver or aluminum, may be used as a material for the detection wiring. The detection wiring formed of wiring of the metal material (hereinafter may be referred to as “metal wiring”) may have a reduced resistance, but the metal wiring is opaque, so that the metal wiring is easily visually identified. There is a technique for making the metal wiring thin and in a mesh pattern to reduce the visibility of the metal wiring and to increase transmittance of the touch screen. Patent Document 2 discloses a projected capacitive touch screen formed of thin metal wiring.
When the thin metal meshed wiring is disposed closely in the operation region of the touch screen, a parasitic capacitance between the detection column-direction wiring and the detection row-direction wiring (hereinafter may be referred to as “line capacity”) increases significantly. This results in harmful effects, for example, an increase in wiring delay and an increase in noise.
The wiring delay may be relieved to some extent by reducing wiring resistance. Patent Document 3, for example, discloses a technique for reducing wiring resistance to relieve wiring delay.
The touch screen disclosed in Patent Document 3 is formed of the detection row-direction wiring and the detection column-direction wiring, and each of which has a zigzag pattern of thin straight metal wiring, and thus both of low resistance and reduced line capacity are achieved.
In the touch screen disclosed in Patent Document 3, row-direction bundle wiring includes a plurality of detection row-direction wires that extend substantially in the row direction and are electrically connected to each other, and column-direction bundle wiring includes a plurality of detection column-direction wires that extend substantially in the column direction and are electrically connected to each other. This allows uniform detection of a touch capacitance including a capacitance between an indicator such as a finger and the detection row-direction wiring and a capacitance between the indicator and the detection column-direction wiring.
However, as disclosed in Patent Document 1 to Patent Document 3, a display apparatus on which the touch screen is mounted has a thickness and a weight increased because the detection wiring is formed on a transparent substrate other than a display panel and the transparent substrate is located on the display panel. The increase in the thickness and weight is the important challenge to mobile phones that need to be thin and light.
Patent Document 4 and Patent Document 5 disclose, as a means of reducing a thickness and a weight of a display apparatus on which a touch screen is mounted to apply the display apparatus to a mobile phone or the like, techniques for integrating the functions of the touch screen inside the display panel or on the surface of the display panel. The technique for integrating the touch screen in the display panel as in Patent Document 4 is referred to as an in-cell technique, and the technique for integrating the touch screen on the surface of the display panel as in Patent Document 5 is referred to as an on-cell technique.